The electronics industry continues to rely upon advances in semiconductor technology to realize higher-functioning devices in more compact areas. For many applications, realizing higher-functioning devices requires integrating a large number of electronic devices into a single silicon wafer. As the number of electronic devices per given area of the silicon wafer increases, the manufacturing process becomes more difficult.
A large variety of semiconductor devices have been manufactured having various applications in numerous disciplines. Such silicon-based semiconductor devices include, among others, metal-oxide-semiconductor (MOS) transistors, such as p-channel MOS (PMOS), n-channel MOS (NMOS) and complimentary MOS (CMOS) transistors, bipolar transistors, and BiCMOS transistors. Each of these semiconductor devices generally includes a semiconductor substrate on which a number of active devices are formed. The particular structure of a given active device can vary between device types. For example, in MOS transistors, an active device generally includes source and drain regions and a gate electrode that modulates current between the source and drain regions.
Elements in semiconductor devices are typically formed in the silicon through the use of well-known deposition, photolithography and etching techniques. The processing of a silicon wafer typically includes a coating step in which a photoresist solution is applied to the wafer. The purpose of the coating step is to apply a uniform polymeric film of selected thickness onto the wafer. This technique is commonly known as spin coating, which involves dispensing the photoresist solution onto the wafer and rapidly spinning the wafer. Spinning the wafer serves to dry the photoresist into a solid or gel layer having the desired film thickness.
The dispensing step is performed by distributing photoresist solution over the entire wafer before the wafer is spun (static dispense) or by applying a small quantity of solution near the center of the wafer while spinning the wafer to distribute the solution (dynamic dispense). During the dispensing step, it is desired to distribute the solution uniformly onto the wafer to allow the formation of a coat of uniform thickness during the spin step. The dispensing apparatus should be maintained at a pre-defined, relatively close distance from the wafer to prevent splashing of the solution. Advanced photoresist dispensing methods have varied wafer rotation speed during chemical delivery of the spin coating cycle in order to achieve uniform photoresist coatings with minimal loss in photoresist material. However, this approach has not always been sufficiently precise in forming the photoresist layer.
In an effort to improve these systems, a light source has been included in the system that is directed at the substrate so that light is reflected up for detection by a camera. Variations in the wafer surface lead to variations in the light reflected. When the camera senses that the substrate is dark (less light reflected up to the camera), more photoresist material is dispensed. The opposite is also true; less photoresist is dispensed if the light reflected off the substrate is too bright. However, the process for depositing photoresist is problematic when the light reflected from the substrate is too dim or the light appears washed out. Variations in wafer surface reflectivity have made it difficult to reproduce from one wafer lot to another, a photoresist layer on a wafer substrate that has a uniform thickness across the wafer surface while at the same time minimizing the required amount of photoresist being dispensed.
Accordingly, there has been a long-standing need for semiconductor manufacturing processes that can overcome the aforementioned disadvantages of the prior art.